A photocurable coating composition has widely been used as an excellent coating material in various industrial fields, since the composition functions as a coating material which is quickly hardened by irradiation of an ultraviolet ray or the like, and an energy requirement necessary for forming a coating film is low.
For a coating material, physical properties and appearance of a coating film formed using the material are generally important. Also, for a photocurable coating composition, it is desired that a coating film formed using the composition has, for example, excellent elongation. It is also desired for a photocurable coating composition to be excellent in handling properties and curability.
In order to cope with such a requirement, Patent Document 1 discloses an invention for obtaining a coating film having improved elongation. In the invention, a compound having a specific structure typified by urethane acrylate is used as a constituent component of a photocurable coating composition. Specifically, the document discloses a photocurable resin composition containing (A) a urethane compound, (B) a photopolymerizable monomer having a reactive unsaturated group and (C) a photopolymerization initiator for a reaction, wherein the urethane compound is obtained by mixing (a) an isocyanate compound having two isocyanate groups in a molecule, (b) polyoxyalkylene glycol having a number average molecular weight of more than 1,000 and not more than 2,500, (c) an alkylene glycol or a polyoxyalkylene glycol having a number average molecular weight of not less than 62 and not more than 1,000, and (d) an ethylenical unsaturated compound having a hydroxyl group so that a ratio of the number of moles of the component (b) relative to the number of moles of the component (c) becomes not less than 1, and also an equivalent ratio of a hydroxy group of the components (b), (c) and (d) relative to an isocyanate group of the component (a) becomes 1.0 to 1.1.
Patent Document 2 discloses an invention in which a fine polymer particle having a core-shell structure is used as a constituent component of a photocurable coating composition. The fine polymer particle used herein, i.e. a core-shell type fine polymer particle, is a commercially available powdered core-shell type fine polymer particle. The document discloses preferable particle diameter of a core-shell type fine polymer particle, but neither discloses nor suggests a dispersion state and a dispersion particle diameter of a core-shell type fine polymer particle in a photocurable coating composition.
Specifically, in the invention described in Patent Document 1, it is intended to obtain a coating film having desired physical properties by using a copolymer of (B) a photopolymerizable monomer having a reactive unsaturated group and (A) an urethane compound as a polymer constituting the coating film. On the other hand, in the invention described in Patent Document 2, it is intended to obtain a coating film having desired physical properties by admixing a core-shell type fine polymer particle with a copolymer constituting the coating film without changing the structure of the copolymer.